Isaac reorganizing as it blows through the Lesser Antilles

By: Dr. Jeff Masters , 8:34 PM GMT on August 22, 2012

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Tropical Storm Isaac continues to maintain top winds of just 45 mph as its center prepares to move through the Lesser Antilles islands late this afternoon and early this evening. The entire Lesser Antilles chain of islands is receiving heavy rains from Isaac, with Martinique picking up 1.46" of rain as of 2 pm EDT, and St. Lucia receiving 1.49". However, Isaac is not yet generating much in the way of tropical storm-force winds, and none of the islands had received winds in excess of 30 mph as of 4 pm EDT. During their storm penetration to obtain their 2 pm EDT center fix, an Air Force Reserve aircraft measured top surface winds of just 40 mph, and a central pressure of 1004 mb. Top winds at their 5000 foot flight altitude were 49 mph. Isaac is undergoing significant changes to its structure. The plane found the center had become a broad, elongated oval that extended 40 miles from NW to SE. The old center, fixed at 2 pm near 16.1°N, and closer to the dry air to Isaac's north, is being challenged for dominance by a new center that is attempting to form near a burst of heavy thunderstorms at 15.5°N. The Hurricane Hunters' latest fix at 3:50 pm EDT put the center near 15.9°N, a southwards shift of about 17 miles. The resulting battle between centers is giving Isaac a rather odd spiral rectangular shape, as seen on visible satellite loops. The Hurricane Hunters did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. A large area of dry air to the north of the storm, as seen on water vapor satellite loops, continues to interfere with development, and Isaac will not be able to begin strengthening until it resolves the battle between it two competing centers, and casts out the dry air infiltrating it. This is going to take at least a day, since Isaac is a very large storm, and it takes more time to spin up a big chunk of the atmosphere. Radar imagery from Barbados and Martinique show plenty of heavy rain showers, mostly on the south side of Isaac where it is moister. There has been a modest increase in spiral banding since this morning.


Figure 1. Afternoon satellite image of Isaac, showing its odd spiral rectangle shape.

Latest model runs for Isaac
The latest set of 12Z (8 am EDT) model runs are very similar to the previous set of runs, which I discussed in detail in this morning's post. The models show a westward track to a point on the south coast of Hispaniola. Isaac's center shift to the south may require some modest adjustments to the south and west for the models. This would result in the storm spending a few hours less time over Hispaniola, and more time over or just south of Cuba. This would slightly decrease the risk to the Dominican Republic, the east coast of Florida, and the Bahamas, but increase the risk to the west coast of Florida. The ECMWF--our best performing model over the past two years--continues to be an outlier among the models. It predicts that Isaac will track just south of Cuba, cross the western tip of Cuba on Monday, then head north towards an eventual landfall in Louisiana. However, this model is keeping Isaac weaker than the other models, and thus predicts the storm will have a weaker response to the trough of low pressure over the Southeast U.S. If the official NHC intensity forecast is right and Isaac becomes a hurricane on Thursday, the more southerly track of the ECMWF is not going to verify, and Isaac will spend considerable time over Cuba on Saturday and Sunday. Where Isaac pops off the coast of Cuba will be critical in determining its future path and intensity, and at this point, we don't know it its more likely that Isaac will go up the east coast of Florida, the west coast, or straight up the peninsula over land. At this point, I'd put the odds at:

40% chance of a track through the Gulf of Mexico, west of Florida
25% chance of a landfall in South Florida, and a track mostly over the Florida Peninsula
35% chance of a track along the east coast of Florida

Which model should you trust?
Wunderground provides a web page with computer model forecasts for many of the best-performing models used to predict hurricane tracks. So which is the best? The best forecasts are made by combining the forecasts from three or more models into a "consensus" forecast. Over the past decade, NHC has greatly improved their forecasts by relying on consensus forecast models made using various combinations of the GFS, GFDL, NOGAPS, UKMET, HWRF, and ECMWF models. If you average together the track forecasts from these models, the NHC official forecast will rarely depart much from it, and the NHC forecast has been hard to beat over the past few years. The single best-performing model over the past two years has been the ECMWF (European Center model). The GFS model has been a close second. You can view 7-day ECMWF and 16-day GFS forecasts on our wundermap with the model layer turned on. Ten-day ECMWF forecasts are available from the ECMWF web site. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on our computer model forecast page that plots positions from the other major models. As seen in Figure 2, the HWRF and GFDL were well behind the ECMWF and GFS in forecast accuracy in 2011, but were still respectable. The BAMM model did very well at 4 and 5-day forecasts. The UKMET, NOGAPS, and CMC models did quite poorly compared to the ECMWF, GFS, GFDL, and HWRF. For those interested in learning more about the models, NOAA has a great training video (updated for 2011.)


Figure 2. Skill of computer model forecasts of Atlantic named storms 2011, compared to a "no skill" model called "CLIPER5" that uses just climatology and persistence to make a hurricane track forecast (persistence=a storm will tend to keep going in the direction it's current going.) OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; NOGAPS=Navy Operational Global Prediction System model; UKMET=United Kingdom Met Office model; ECMWF=European Center for Medium Range Weather Forecasting model; TVCA=one of the consensus models that lends together several of the above models; CMC=Canadian Meteorological Center (GEM) model; BAMM=Beta Advection Model (Medium depth.) Image credit: National Hurricane Center 2011 verification report.

I'll have a new post in the morning.

Jeff Masters

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3201. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 36899
3200. VR46L
I dont think TD 10 is going to make it ...





but the 10% a different story....



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Quoting shfr173:
Can someone explain what is pulling Isaac southward?
Good Morning.

Doesn't appear to be weakness "pulling" it from the south but rather "strength" pushing it from the north.
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Quoting RTSplayer:
1000mb



990mb



In case anyone's wondering, the Euro says this needs to get all the way around behind the high that is over Belize and the Yucatan presently.

There's actually STILL a slight southerly component in steering, so expect another southern correction 3 hours from now.


if isaac were a strong storm, he would've blown thru the north carib. and missed the conus entirely with that steering.
Member Since: August 22, 2006 Posts: 2 Comments: 1975
Quoting shfr173:
Can someone explain what is pulling Isaac southward?


There's a blow up of convection to the south which creates the optical illusion of southwards motion.

However, the NHC have moved the latitude of the COC southwards slightly. This is probably not indicative that the storm has moved south, more that the COC has relocated south.
Member Since: Posts: Comments:
Quoting AussieStorm:
TROPICAL STORM ISAAC ADVISORY NUMBER 9
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
500 AM AST THU AUG 23 2012

...AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT FINDS ISAAC SLIGHTLY
WEAKER AND FARTHER SOUTH...


SUMMARY OF 500 AM AST...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...15.3N 64.0W
ABOUT 255 MI...410 KM SSE OF SAN JUAN PUERTO RICO
ABOUT 370 MI...595 KM ESE OF ISLA SAONA DOMINICAN REPUBLIC
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...W OR 270 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...1004 MB...29.65 INCHES

Slowed down 8mph.... This will give Isaac time to consolidate and develop.


and 15.3, can you believe it? yesterday we had 'im pegged up almost to the 16's and over. i did so much hurricane prep work.

>:|
Member Since: August 22, 2006 Posts: 2 Comments: 1975
That is one healty looking 40Mph storm
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Darn. Looks like it's time to refill the gas cans for the generator. Nobody yet paying too much attention to Isaac here in Panama City Beach.
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1000mb



990mb



In case anyone's wondering, the Euro says this needs to get all the way around behind the high that is over Belize and the Yucatan presently.

There's actually STILL a slight southerly component in steering, so expect another southern correction 3 hours from now.
Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting shfr173:
Can someone explain what is pulling Isaac southward?


hes not being pulled southward so much as hes reforming his center. if anything, storms are pulled poleward upon intensification or drift with the trades. as you know, its still a 40kt storm.
Member Since: August 22, 2006 Posts: 2 Comments: 1975
Quoting AussieStorm:
TROPICAL STORM ISAAC ADVISORY NUMBER 9
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
500 AM AST THU AUG 23 2012

...AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT FINDS ISAAC SLIGHTLY
WEAKER AND FARTHER SOUTH...


SUMMARY OF 500 AM AST...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...15.3N 64.0W
ABOUT 255 MI...410 KM SSE OF SAN JUAN PUERTO RICO
ABOUT 370 MI...595 KM ESE OF ISLA SAONA DOMINICAN REPUBLIC
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...W OR 270 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...1004 MB...29.65 INCHES

Slowed down 8mph.... This will give Isaac time to consolidate and develop.
Its not moving at 12mph!!!! NHC TYPO!
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3189. emguy
Quoting emguy:


It (EURO-ECMWF) likely will move further west next run. The weakness in the ridge is not conducive for a storm to go that far west. The model have consistently shifted further east for four (4) continuous runs and will likely continue to do so until moving east over the extreme eastern gulf (east of Apalachicola, FL).


I correct this...further east.
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Quoting bigwes6844:
was
2:00 AM AST Thu Aug 23
Location: 15.5�N 63.5�W
Moving: W at 20 mph
Min pressure: 1003 mb
Max sustained: 45 mph

now
5:00 AM AST Thu Aug 23
Location: 15.3�N 64.0�W
Moving: W at 12 mph
Min pressure: 1004 mb
Max sustained: 40 mph
Isaac is NOT moving 12mph!!! LOL must have been a typo by the NHC
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Good morning folks.
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3186. emguy
Quoting PensacolaDoug:



God I hope not!


It (EURO-ECMWF) likely will move further west (TYPO: CORRECT TO READ FURTHER EAST) next run. The weakness in the ridge is not conducive for a storm to go that far west. The model have consistently shifted further east for four (4) continuous runs and will likely continue to do so until moving east over the extreme eastern gulf (east of Apalachicola, FL).
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3185. ackee
Quoting shfr173:
Can someone explain what is pulling Isaac southward?
IT seem like the HIGH may be a bit stronger than what was expect THAT was what The Ecmwf was showing all along guess we see
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Quoting shfr173:
Can someone explain what is pulling Isaac southward?


I dunno, I see a northward pull in the last couple of hours.
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3183. ackee
Quoting quante:
again, a tad westward, but still, if it hits high mountains, could pop out anywhere in Florida Bay. It does look a bit south of forecast points, so it may miss, stay south, and then the ECMWF may have it right. Especially with the lack of organization, not sure when the trough is coming, but storm may be too weak to be influenced by it. Stay tuned. Glad I don't work at NHC this week.
Agree I expect a major change in track that will be more inline with Ecmwf
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Quoting ackee:
I think the Ecmwf may end up being right



God I hope not!
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3181. shfr173
Can someone explain what is pulling Isaac southward?
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I maintain the fact that there is another center.
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3179. quante
again, a tad westward, but still, if it hits high mountains, could pop out anywhere in Florida Bay. It does look a bit south of forecast points, so it may miss, stay south, and then the ECMWF may have it right. Especially with the lack of organization, not sure when the trough is coming, but storm may be too weak to be influenced by it. Stay tuned. Glad I don't work at NHC this week.
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3178. DDR
Good morning
reporting in from Trinidad,heavy rain has been falling steadily for more than half an hour in the devastated north western peninsula,with the sun will come the devastation once again.
Member Since: April 27, 2007 Posts: 14 Comments: 1687
3177. tramp96
Quoting AussieStorm:

ECMWF 168hrs...









Do you have a link for those graphs?
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Quoting cntrclckwiseSpenn:




I would be less worried if the forecast model tracks were closer/on-top of west florida to central Florida as they were a day or two ago. The models have it in the panhandle, and well, the actual overall tracks of storms tend to trend slightly to the east of the models. West Coast florida storms are fairly rare, but the timing and strength of a trough coming down as the storm arrives, and any weakness in the bermuda high to the east might just be enough to nudge it back to the west coast of Florida. Whether its north or south of the Bay area remains to be seen, but I'm getting nervous. usually more often then not storms find ways to miss the bay area though, so well see, and cross our fingers.
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Quoting AussieStorm:

ECMWF 168hrs...








Too many days out to think this will happen - I feel much better now Thank You!- I live in Fort Walton Beach
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Quoting AussieStorm:

Me too. or a combo of the ECMWF and the GFS.






I would be less worried if the track was closer to west florida. The models have it in the panhandle, and well, the actual overall tracks of storms tend to trend slightly to the east of the models. West Coast florida storms are fairly rare, but the timing and strength of a trough coming down as the storm arrives, and any weakness in the bermuda high to the east might just be enough to nudge it back to the west coast of Florida. Whether its north or south of the Bay area remains to be seen, but I'm getting nervous. usually more often then not storms find ways to miss the bay area though, so well see, and cross our fingers.
Member Since: Posts: Comments:
TROPICAL STORM ISAAC ADVISORY NUMBER 9
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012
500 AM AST THU AUG 23 2012

...AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT FINDS ISAAC SLIGHTLY
WEAKER AND FARTHER SOUTH...


SUMMARY OF 500 AM AST...0900 UTC...INFORMATION
----------------------------------------------
LOCATION...15.3N 64.0W
ABOUT 255 MI...410 KM SSE OF SAN JUAN PUERTO RICO
ABOUT 370 MI...595 KM ESE OF ISLA SAONA DOMINICAN REPUBLIC
MAXIMUM SUSTAINED WINDS...40 MPH...65 KM/H
PRESENT MOVEMENT...W OR 270 DEGREES AT 12 MPH...19 KM/H
MINIMUM CENTRAL PRESSURE...1004 MB...29.65 INCHES

Slowed down 8mph.... This will give Isaac time to consolidate and develop.
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was
2:00 AM AST Thu Aug 23
Location: 15.5N 63.5W
Moving: W at 20 mph
Min pressure: 1003 mb
Max sustained: 45 mph

now
5:00 AM AST Thu Aug 23
Location: 15.3N 64.0W
Moving: W at 12 mph
Min pressure: 1004 mb
Max sustained: 40 mph
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3170. wn1995
New adv... down to 40 mph.. no surprise... also further south... no surprise. Waiting on the track.
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This must be serious the NHC hasnt updated at all yet
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Quoting uncwhurricane85:


if it never develops down in the carib it will never make it to the gulf, it will run into yucatan


there is a enough spin there to keep it going, plus the warmer waters below cuba will likely make it a little stronger even if it doesnt cross as a strong hurricane.
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I can't believe what the GFS is suggesting in the WPAC. Watch this loop and have a look. It would be amazing to see this happen.
The ECMWF is on board also.
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i bet you a lot of people F5!!!!!! like crazy on the NHC site!! really nhc u want us to wait until 5 it kool. lol
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Quoting ackee:
I think the Ecmwf may end up being right

Me too. or a combo of the ECMWF and the GFS.



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Quoting ackee:
I think Issac centre is well south of the NHC and most of the model track I think the track need to Ajusted more south and west


NHC Track loop.
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3163. ackee
Quoting AussieStorm:

ECMWF 168hrs...








I think the Ecmwf may end up being right
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Quoting ackee:
I am beging think THAT models may be too Far NORTH with Isaac I buying the Ecmwf track with Issac I dont see the cantre further south

ECMWF 168hrs...








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3161. ackee
I think Issac centre is well south of the NHC and most of the model track I think the track need to Ajusted more south and west
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RECONNAISSANCE AIRCRAFT FINDS THE CENTER OF ISAAC IS REFORMING FARTHER SOUTH. Also Isaac's movement is still westerly at 20mph.
That would mean Isaac has more of a chance of being a GOM storm.
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Quoting justsouthofnola:
looks more like one center to me..... but its got a wsw movement still

Link


Look at the shortwave.
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3158. emguy
Overall...

1.) Isaac relocated and is getting his act together.
2.) He actually maybe looks more ragged to some than he has in the past, but it is likely he is getting settled in and is actually much better organized then ever before.
3.) Around the new center, we have banding going on. Banding is defined by the lines. Recon center is the "X".

Actually, this is a level of organization I dislike, because the overall structure suggests large, strong, healthy storm that wants to be a well developed hurricane in the deep tropics.

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looks more like one center to me..... but its got a wsw movement still

Link
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3156. ackee
Quoting yonzabam:
Still going west and still south of forecast points.

The models all have it tracking WNW from this point. Somehow, I don't think that's going to happen.
I am beging think THAT models may be too Far NORTH with Isaac I buying the Ecmwf track with Issac I dont see the cantre further south
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Quoting emguy:
This time last night...I posted the following forecast thinking. The red lines were "my cone of thinking". I also had an orange line, and expresssed on the blog that this was an area that was "out of bounds" for my forecast cone, and not part of my forecast, but there to denote some respect for the Euro.

Last Night:



With tonight's likely relocation of the center, everything in that previous map may now be out of bounds...These are the adjustments...Florida is still the heart of it...but the look is different. The map is based on everything I can consider this evening (which is a lot of info):



Hi Everyone :)
With the center relocation to the south I think that if Isaac does make the turn toward the NW he will "rebound" off the mountains/south coast of Hispaniola and head west toward us but at this point I am not very convinced that the turn will happen at all so am rather concerned because NO ONE in Jamaica is preparing and in a scenario where the model track hasn't been bee-lining toward us from the start it is usually the very last minute before a watch or warning goes into effect so I really wish there was more debate about the islands and less on mainland US landfall next week?!
Honestly, I almost hope it does hit us and not Haiti as at least we have forestation and roofs over our heads here but I would greatly appreciate opinions on the possibility of a direct hit to Jamaica Friday-Saturday...Thanks!
One Love
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Quoting emguy:


Boy oh boy Joe, that is a tough call...but the waters around Jamaica and south of Central Cuba are hot enough to intensify a Hurricane at a good clip.


Thanks bud!
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Quoting RussianWinter:
I'm looking at shortwave and I am not buying just one center, for the last time.


been saying that all night!!! its driving me nuts! there are 2 centers
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I'm looking at shortwave and I am not buying just one center, for the last time.
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Quoting cntrclckwiseSpenn:


it will definitely develop in the GOM. Just if its a stronger storm crossing, i think it will take longer to redevelop if it was knocked down to a weak storm, than a weak storm that crosses and then has a broad circulation still intact allowing it to regenerate more quickly then one the has its circulation displaced.


if it never develops down in the carib it will never make it to the gulf, it will run into yucatan
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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